It has long been known that thermoplastic materials may be "oriented", i.e., the molecules of the material are oriented or aligned and then frozen into their aligned relationship. Such orientation increases the physical properties of the material in the direction of orientation.
For example, in "stretch oriented" filaments, an extruded strand is stretched axially, and the tensile strength of the strand is increased by several orders of magnitude related to the amount of stretching. Similarly, sheet material can be "bi-axially oriented" by stretching the sheet longitudinally and transversely. The physical strength of such a bi-axially oriented sheet is again increased by several orders of magnitude. In a similar manner, blow molded articles can be "multi-directionally oriented" by simply blowing the material into a final article, the stretching of the parison or pre-form upon blowing to the final shape accomplishing the orientation. In a blown article, the physical properties are increased upon orientation, and the greater resistance of the wall to the transmission of vapor there through is also known to be a consequence of multi-directional orientation.
In order to accomplish orientation in thermoplastic articles by presently known techniques, the material must be stretched while it is at a temperature conducive to orientation, i.e., a temperature at which the molecules are sufficiently mobile to be oriented and yet are not so mobile as to immediately lose the orientation which has been induced therein. Some orientation is necessarily incuded into any thermoplastic material as it is issued from an orifice, as in normal extrusion or injection molding operations, but the material is linearly oriented in the direction of flow only, and the material is at such an elevated temperature that the orientation is dissipated by random rearrangements of the thermally active molecules. It has been determined that the thermal decay of orientation is inversely proportional to the temperature of the materials at the time the orientation stress is induced. In other words, at normal extrusion temperatures, on the order of 400.degree. F., the orientation induced during flow through the orifice is dissipated by molecular rearrangement in an extremely short period of time, usually on the order of 0.1 seconds.
As a result, present orientation procedures are carried out after the thermoplastic material has issued from the orifice and has been cooled exteriorly of the orifice to a temperature at which the molecules are less mobile and the orientation dissipation times are greatly increased. For example, in stretch orientation, the extruded material is cooled in a water bath and then reheated in a "orienting oven" to a temperature substantially less than extrusion temperature, and the extruded, chilled and reheated strand is then stretched. In blow molding, an injection molded parison is chilled in the open air after injection and is then blown. Alternatively, an extruded parison is chilled to a rigid state and then reheated to a temperature substantially less than extrusion temperature, so that orientation can be obtained during blowing at the lower temperature. Orientation is normally not accomplished in injection molding, since the material is injected into the mold at or near its melt temperature, at which temperature any orientation obtained at the orifice is dissipated before the mold filling and chilling operation is complete.